Swedish EPA (Naturvårdsverket) noise level guide-lines suggest that equivalent A-weighted sound pressure levels (SPL) must not exceed 40 dBA at residents. Thus, in the planning of new wind farms and their location it is crucial to estimate the disturbance it may cause to nearby residents. Wind turbine noise emission levels are guaranteed by the wind turbine manufacturer only under ice-free conditions. Thus, ice accretion on wind turbine may lead to increased wind turbine noise resulting in noise levels at nearby residents to exceed 40 dBA SPL. The purpose of the project is to evaluate the effect on wind turbine noise emission due to ice accretion. This, by trying to quantify the ice accretion on rotor blades and correlate it to any change in noise emission. A literature study shows that the rotor blades are to be considered the primary noise source. Hence, ice accretion on rotor blades are assumed to be the main influence on noise character. A field study is performed in two parts; as a long term measurement based on the method out-lined by IEC 61400-11 and as a short term measurement in strict accordance with IEC 61400-11. These aim to obtain noise emission levels for the case of icing conditions and ice-free conditions (reference conditions) as well as background noise levels. An analysis is performed, which sets out to correlate ice measurements with wind turbine performance and noise emission. Data reduction procedures are performed according to IEC 61400-11.The apparent sound power levels are evaluated. This is performed for the case of icing conditions as well as for the case of ice-free onditions. A statistical evaluation of icing event is carried out. The results show that ice accretion on wind turbine (rotor blades) may lead to drastically higher noise emission levels. The sound power levels show an average increase of 10.6 dB at 8 m/s. However, this can occur at all wind speeds from 6 m/s to 10 m/s. Higher levels of noise, (55 to 65 dBA SPL) may be caused by very small amounts of ice accretion. Occurrences of higher levels of noise, in the range of 50 to 65 dBA SPL, are not common. Noise levels exceeding 50 dBA SPL are to expected 10.3 % of the time during the winter or 3 % of the time during one year. Correlation between measured ice accumulation and noise level is weak apart from large amounts of ice. This due to statistical noise. Taking into account the noise level guide-lines of 40 dBA SPL at residents, as is recommended by Swedish EPA (Naturvårdsverket), the increased levels of windturbine noise under icing conditions may force the power production to a halt.

This thesis deals with the stability of particle laden flows. Both modal and non-modal linear analyses have been performed on two-way coupled particleladen flows, where particles are considered spherical, solid and either heavy or light. When heavy particles are considered, only Stokes drag is used as interaction term. Light particles cannot be modeled with Stokes drag alone, therefore added mass and fluid acceleration are used as additional interaction forces. The modal analysis investigates the asymptotic behavior of disturbances on a base flow, in this thesis a pressure-driven plane channel flow. A critical Reynolds number is found for particle laden flows: heavy particles increase the critical Reynolds number compared to a clean fluid, when particles are not too small or too large. Neutrally buoyant particles, on the other hand, have no influence on the critical Reynolds number. Non-modal analysis investigates the transient growth of disturbances, before the subsequent exponential behavior takes over. We investigate the kinetic energy growth of a disturbance, which can grow two to three orders of magnitude for clean fluid channel flows. This transient growth is usually the phenomenon that causes transition to turbulence: the energy can grow such that secondary instabilities and turbulence occurs. The total kinetic energy of a flow increases when particles are added to the flow as a function of the particle mass fraction. But instead of only investigating the total energy growth, the non-modal analysis is expanded such that we can differentiate between fluid and particle energy growth. When only the fluid is considered in a particle-laden flow, the transient growth is equal to the transient growth of a clean fluid. Besides thes Stokes drag, added mass and fluid acceleration, this thesis also discusses the influence of the Basset history term. This term is often neglected in stability analyses due to its arguably weak effect, but also due to difficulties in implementation. To implement the term correctly, the history of the particle has to be known. To overcome this and obtain a tractable problem, the square root in the history term is approximated by an exponential. It is found that the history force as a small effect on the transient growth. Finally, Direct numerical simulations are performed for flows with heavy particles to investigate the influence of particles on secondary instabilities. The threshold energy for two routes to turbulence is considered to investigate whether the threshold energy changes when particles are included. We show that particles influence secondary instabilities and particles may delay transition. / QC 20111013

Undesired air entrainment in bottom outlet conduits of dams may cause pressure transients, leading to conduit vibrations, blowback, discharge pulsation and even cavitation, and jeopardize the operational safety. Due to design limitations or construction costs, it is impossible to create an air free environment in a pressurized pipe. Therefore, it is essential to understand the air transport in enclosed pipes in order to provide guidance in bottom outlet design and operation. The commonly used criterion of the air-pocket movement in pipe flow is the water flow velocity for starting moving an air pocket, the so-called critical velocity. In this thesis, the classical Volume of Fluid (VOF) model combined with the k-ε turbulence model is adopted for the computation of the critical velocity of a 150-mm pipe. The computed critical velocities are compared with the experimental results. The governing parameters investigated in this study include pipe slope and diameter, wall shear stress and air-pocket volume. Meanwhile, the carrying capacity (air-pocket velocity/ flow velocity) at all pipe slopes are analyzed. The simulation results of air pockets with different volumes in the bottom outlet conduit of Letten Dam in Sweden are presented in this study. Moreover, experimental study was conducted to measure the critical velocity for a 240-mm Plexiglas pipe. The results are in agreement with the experiments performed by HR Wallingford (HRW) in 2003 in terms of the effects of pipe slope and air-pocket volume; however, the critical Froude pipe number is slightly smaller in this study. In rough pipes, a larger critical velocity is required compared with that in the smooth pipe. The removal mechanism in the rough pipe involves the successive loss of air caused by turbulence. This explains that the air-pocket size, with the dimensionless air-pocket volume n < 0.015, has little impact on the critical velocity for the rough pipe. In addition, roughness has little impact on the air-pocket velocity when it moves upstream in the downward inclined pipe. The trapped air bubbles most likely remain permanently in the rough pipe. / QC 20120110

This thesis deals with turbulent flows in 90 degree curved pipes of circular cross-section. The flow cases investigated experimentally are turbulent flow with and without an additional motion, swirling or pulsating, superposed on the primary flow. The aim is to investigate these complex flows in detail both in terms of statistical quantities as well as vortical structures that are apparent when curvature is present. Such a flow field can contain strong secondary flow in a plane normal to the main flow direction as well as reverse flow. The motivation of the study has mainly been the presence of highly pulsating turbulent flow through complex geometries, including sharp bends, in the gas exchange system of Internal Combustion Engines (ICE). On the other hand, the industrial relevance and importance of the other type of flows were not underestimated. The geometry used was curved pipes of different curvature ratios, mounted at the exit of straight pipe sections which constituted the inflow conditions. Two experimental set ups have been used. In the first one, fully developed turbulent flow with a well defined inflow condition was fed into the pipe bend. A swirling motion could be applied in order to study the interaction between the swirl and the secondary flow induced by the bend itself. In the second set up a highly pulsating flow (up to 40 Hz) was achieved by rotating a valve located at a short distance upstream from the measurement site. In this case engine-like conditions were examined, where the turbulent flow into the bend is non-developed and the pipe bend is sharp. In addition to flow measurements, the effect of non-ideal flow conditions on the performance of a turbocharger was investigated. Three different experimental techniques were employed to study the flow field. Time-resolved stereoscopic particle image velocimetry was used in order to visualize but also quantify the secondary motions at different downstream stations from the pipe bend while combined hot-/cold-wire anemometry was used for statistical analysis. Laser Doppler velocimetry was mainly employed for validation of the aforementioned experimental methods. The three-dimensional flow field depicting varying vortical patterns has been captured under turbulent steady, swirling and pulsating flow conditions, for parameter values for which experimental evidence has been missing in literature. / QC 20120425

Rotating-disk flow has been investigated not only as a simple model of cross flow instability to compare with swept-wing flow but also for industrial flow applications with rotating configurations. However the exact nature of laminar-turbulent transi- tion on the rotating-disk flow is still major problem and further research is required for it to be fully understood, in particular, the laminar-turbulent transition process with absolute instability. In addition the studies of the rotating-disk turbulent boundary- layer flow are inadequate to understand the physics of three-dimensional turbulent boundary-layer flow. In present thesis, a rotating-rotating disk boundary-layer flow has been inves- tigated experimentally using hot-wire anemometry. A glass disk with a flat surface has been prepared to archieve low disturbance rotating-disk environment. Azimuthal velocity measurements using a hot-wire probe have been taken for various conditions. To get a better insight into the laminar-turbulent transition region, a new way to describe the process is proposed using the probability density function (PDF) map of azimuthal fluctuation velocity. The effect of the edge of the disk on the laminar-turbulent transition process has been investigated. The disturbance growth of azimuthal fluctuation velocity as a function of Reynolds number has a similar trend irrespective of the various edge conditions. The behaviour of secondary instability and turbulent breakdown has been in- vestigated. It has been found that the kinked azimuthal velocity associated with secondary instability just before turbulent breakdown became less apparent at a cer- tain wall normal heights. Furthermore the turbulent breakdown of the stationary mode seems not to be triggered by its amplitude, however, depend on the appearance of the travelling secondary instability. Finally, the turbulent boundary layer on a rotating disk has been investigated. An azimuthal friction velocity has been directly measured from the azimuthal velocity profile in the viscous sub-layer. The turbulent statistics normalized by the inner and outer sclaes are presented. / QC 20120529

This work investigates the fluid flow in a cylindrical millilitre vial stirred by a magnetic stirred bar using Computational Fluid Dynamic (CFD). Stirred millilitre vials are used to study nucleation phenomena and crystallization as an outline of literature study of nucleation and crystallization phenomena and the role of stirring in this process. The baffle free vial was meshed with around 500,000 cells. To simulate the stirring a rotary frame and moving walls were used. Stirring speeds were between 100 and 1000 rpm where considered, correspondently to a stirrer Reynolds number between 260 and 2600. For stirring speeds bellow 500 rpm, simulations by both the both laminar flow model and the k-ε model where run, while above 500 rpm only k-ε was used. Results of the two models were very similar indicative the adequacy of k-ε to simulate the flow even at low Reynolds. The flow shows expected circulation pattern with upwards pumping close to side walls and downwards pumping in the centre of cylindrical vial. At 1000 rpm circulation patterns expands up to the top of the vial while at 300 rpm and lower the upper half of the vial is poorly mixed. The average turbulent energy of the flow is very low comparing with the squared stirrer tip speed and the power number decrees with Reynolds number, indicating that the flow is not fully turbulent.

There is a wide variety of tails, fins, scales, riblets and surface coatings, which are used by motile animals in nature. Since organisms currently living on earth have gone through millions of years of evolution, one can expect that their design is optimal for their tasks, including locomotion. However, the exterior of living animals has range of different functions, from camouflage to heat insulation; therefore it is a very challenging task to isolate mechanisms, which are beneficial to reduce the motion resistance of the body. There are two general categories of mechanisms existing in locomotion and flow control. The first is active flow control, when an organism is actively moving some parts or the whole body (exerts energy) in order to modify the surrounding flow field (for example, flapping bird wings). The second is passive flow control, in which an organism has an appendage or a coating, which is not actively controlled (no energy is spent), but is interacting with surrounding flow in a beneficial way. Our aim is to find novel mechanisms for passive flow control. We start by looking at a simple model of an appendage (splitter plate) behind a bluff body (circular cylinder). If a recirculation region forms behind the body, already in this simple system there is a symmetry breaking effect for sufficiently short plates, which passively generates turn and drift of the body. We have found that this effect is caused by the pressure forces in the recirculation region, which pushes the plate away from the vertical in a manner similar to how a straight inverted pendulum falls under the influence of gravity. In order to investigate this symmetry breaking, we developed an extension of the immersed boundary projection method, in which the rigid body dynamics and fluid dynamics are coupled implicitly. The method is capable of solving for particle motion in a fluid for very small density ratios. We also explain our findings by a simple yet quantitative reduced-order model and soap-film experiments. To extend our work, we investigate flow around bodies, which are coated by a porous and elastic material. We have analysed various theoretical approaches to modeling a coating in a continuous manner. We aim to solve the governing equations numerically. We have selected multi-scale expansion approach, of which we present some initial results. / Många djur använder sig av fjäll, päls, hår eller fjädrar för att öka sin förmåga att förflytta sig i luft eller vatten. Evolutionen har främjat ojämna, sträva eller gropiga ytor, vilka har en tendens att minska det totala motståndet som uppstår när en kropp rör sig i vatten eller luft, jämfört med en helt slät och jämn yta.Det finns två kategorier av metoder för manipulering av strömning (så kallad flödeskontroll). Den första är en aktiv metod, där organismer aktivt rör hela eller delar av kroppen (förbrukar energi) för att manipulera omgivande strömningsfält. Den andra metoden är passiv, där organismer har utväxter eller ytbeläggningar som de inte är aktivt har kontroll över (ingen energi förbrukas), men som samverkar med omgivande strömningsfält på ett fördelaktigt sätt. Vårt mål är att hitta nya mekanismer för passiv flödeskontroll.Vi börjar med att studera en enkel modell för hur en utväxt samverkar med en strömmande fluid genom att fästa en platta på en cirkulär cylinder. Om en vak (så-kallad återcirkulationsregion) bildas bakom kroppen, bryts symmetrin i strömningsfältet då plattan är tillräckligt kort. Som en konsekvens av detta roterar kroppen och driver i sidled. Vi visar att detta fenomen orsakas av tryckkrafter i återcirkulationsregionen, som förskjuter plattan från dess vertikala läge. Vi argumenterar att denna mekanism är samma mekanism som får en inverterad pendel att falla under inverkan av gravitation. För att analysera symmetribrytningen, utvecklade vi en numerisk metod (immersed boundary projection method), som implicit kopplar stelkropps- och strömningsdynamik. Med hjälp av denna metod kan vi simulera partiklar i fluider med väldigt låga densitetsskillnader. Våra resultat förklaras även med hjälp av en enkel modell av låg ordning och med hjälp av såphinneexperiment.Som nästa steg i vårt arbete, ämnar vi att studera strömningen kring kroppar som är belagda av tät, porös och elastisk beläggning. Vi har analyserat möjliga tillvägagångssätt för att modellera beläggningar med kontinuumteori. Vi har valt en metod baserad på en flerskalig expansionsmetod, från vilken vi presenterar våra preliminära resultat. / <p>QC 20150119</p>

A focus has been put on the stability characteristics of different flow types existing on air vehicles. Flow passing over wings and different junctions on an aircraft face numerous local features, ranging from different pressure gradients, to interacting boundary layers. Primarily, stability characteristics of flow over a wing subject to negative pressure gradient is studied. The current numerical study conforms to an experimental study conducted by Saric and coworkers, in their Arizona State University wind tunnel experiments. Within that framework, a passive control mechanism has been tested to delay transition of flow from laminar to turbulence. The same control approach has been studied here, in addition to underling mechanisms playing major roles in flow transition, such as nonlinear effects and secondary instabilities. Another common three-dimensional flow feature arises as a result of streamlines passing through a junction, the so called corner-flow. For instance, this flow can be formed in the junction between the wing and fuselage on a plane. A series of direct numerical simulations using linear Navier-Stokes equations have been performed to determine the optimal initial perturbation. Optimal refers to a perturbation which can gain the maximum energy from the flow over a period of time. Power iterations between direct and adjoint Navier- Stokes equations determine the optimal initial perturbation. In other words this method seeks to determine the worst case scenario in terms of perturbation growth. Determining the optimal initial condition can help improve the design of such surfaces in addition to possible control mechanisms. / <p>QC 20130604</p> / RECEPT

The following project has been carried out in collaboration with Abstracta AB (AAB), a company based in Lammhult with specialization in sound landscapes for indoor environments. The company manufactures different types of products with sound-absorbing properties where sustainability and design are in great focus. A new product idea of the company is based on developing an A-classified discreetly designed wall absorber that is considered more as a "wall" than a "wall with mounted absorbent on". The purpose of the project was to develop the following product on a conceptual level. A main research question with two underlying issues was developed that served as a support during the project. Question 1: How can an A-rated discreetly designed sound wall absorber be developed? 1.a: How can an A-rated sound wall absorber be developed? 1.b: How can a sound wall absorber achieve a discreetness in its design? The project followed a product development process where the mainly focus was on concept development and concept testing. The concept was developed based on data collected from a literature study and with the use of a sound absorption calculator. A sound absorption test and an examination form regarding the discrete modeling and design of the absorbent were performed for the final concept that the project resulted in. The project showed that an A-rated discreetly designed sound wall absorber can be developed through a process where design-critical specifications that affect the absorber's discrete design control how the object can be modeled. Via a sound absorption calculator, concepts can be designed that theoretically indicate an A-rate while the design-critical specifications are not exceeded. Based on the concept, a prototype is manufactured that can undergo a sound absorption test to determine the absorbent's actual sound absorption capacity. Finally, an examination is made to analyze whether the absorbent is considered a discrete alternative or not. Based on the project's results, the work process proved to be advantageous as the concept indicated a sound absorption capacity that exceeded the requirements of the A-rating and at the same time obtained a discreet design and modeling. This was achieved in connection with the manufacturing materials achieving their technical specifications and a square meter cost lower than expected. Based on the project's results, the work process proved to be advantageous as the concept indicated a sound absorption capacity that exceeded the requirements of the A-rating and at the same time obtained a discreet design and modeling. This was achieved in connection with the manufacturing materials achieving their technical specifications and a square meter cost lower than expected. In the further development of the final concept, a solution for how the absorbent is to be mounted on the wall must be developed as this was not focused on in the project based on its boundaries. To achieve a valid A-rate, an additional sound absorption test must also be performed where a larger object area of 10–12 m2 is tested. In future work, the concept's discrete modeling and design should also be examined based on physical treatment to review how the examination form's response results relate to this. / Följande projekt utförs i samarbete med Abstracta AB (AAB), ett företag som är stationerat i Lammhult och är specialiserade inom ljudlandskap för inomhusmiljöer. Företaget framställer olika typer av produkter med ljudabsorberande egenskaper där hållbarhet och design även är i stort fokus. En ny produktidé av företaget grundar sig i att utveckla en A-klassificerad diskret utformad väggabsorbent som betraktas mer som en ”vägg” än en ”vägg med uppmonterad absorbent på”. Syftet med projektet var att framta följande produkt på en konceptuell nivå. En huvudsaklig forskningsfråga med två underliggande frågeställningar togs fram som fungerade som ett stöd under projektets gång. Fråga 1: Hur kan en A-klassificerad diskret utformad ljudväggabsorbent utvecklas? 1.a: Hur kan en A-klassificerad ljudväggabsorbent utvecklas? 1.b: Hur kan en ljudväggabsorbent uppnå en diskrethet i sin design? Projektet följde en produktutvecklingsprocess där det största fokuset låg på konceptutvecklingen och koncepttestningen. Koncepten utvecklades utifrån insamlade data från en litteraturstudie och med användning av en ljudabsorptionskalkylator. Ett ljudabsorptionstest och ett undersökningsformulär gällande absorbentens diskreta utformning och design utfördes för det slutgiltiga konceptet som projektet resulterade i. Projektet visade att en A-klassificerad diskret utformad ljudväggabsorbent kan utvecklas genom en process där designkritiska specifikationer som berör absorbentens diskreta utformning styr hur objektet kan formges. Via en ljudabsorptionskalkylator kan koncept designas som teoretiskt indikerar en A-klassning samtidigt som de designkritiska specifikationerna ej överskrids. Utifrån konceptet tillverkas en prototyp som kan genomgå ett ljudabsorptionstest för att fastställa absorbentens verkliga ljudupptagningsförmåga. En undersökning görs slutligen för att analysera huruvida absorbenten betraktas som ett diskret alternativ eller ej. Utifrån projektets resultat visade sig arbetsprocessen vara fördelaktig då konceptet påvisade en ljudabsorptionsförmåga som överträffade A-klassningens krav och samtidigt erhöll en diskret utformning och design. Detta åstadkom i samband med att tillverkningsmaterialen uppnådde sina tekniska specifikationer och en kvadratmeterkostnad lägre än väntat. I fortsatt utveckling av det slutgiltiga konceptet måste en lösning till hur absorbenten ska monteras på väggen tas fram eftersom detta inte fokuserades på i projektet utifrån dess avgränsningar. För att en giltig A-klassificering ska nås måste även ytterligare ett ljudabsorptionstest utföras där en större objektarea på 10–12 m2 testas. I framtida arbete bör även konceptets diskreta utformning och design undersökas utifrån fysiska bemötanden för att granska hur undersökningsformulärets svarsresultat förhåller sig till detta.

Turbulent mixing is present in many pulp and paper processes. It is a particularly important factor in the design and improvements of the paper machine headbox, influencing the final paper structure. During this project, experimental methods to quantify the effect of fibers on turbulent suspension flows have been developed, and then used for studying turbulent mixing in fiber suspensions. A technique that uses microprobes to measure passive scalar mixing of salt for the characterization of turbulent fluctuations in a fiber suspension flow has been developed: Conductivity micro-probes have been built and turbulence measurements have been performed in simple jet and wake flows, studying turbulent mixing between the two streams of pulp suspension, of which one has been doped with salt. A relatively new technique to measure fluid velocity non-intrusively in opaque fluids has also been tested. The technique makes use of ultrasonic pulses to obtain velocity information through the Doppler-shift of reflected pulses. The main efforts reported on in the thesis are focused on method design and development as well as method evaluation.